In the past, engine oil additives contributed to fuel economy in two ways. First, additives supported the development of engine designs that improved fuel economy without compromising performance while maintaining durability over the oil drain interval. Second, new additive technologies and engine oil formulations, tailor-made for oil companies and OEMs, improved fuel economy in their own right.

Today, the potential remains to improve fuel economy even more by moving to low and ultra-low viscosity oils and by redesigning hardware. The optimum approach is a co-engineered solution where engine and lubricant are developed in parallel. However, limiting the approach to fuel economy alone is not sufficient. A holistic viewpoint is needed that considers all performance requirements of future engine oils, keeping in mind that performance cannot be defined based solely on chemical limits.

Meeting the Regs

The industry faces major challenges in the next decade and beyond. All regions are broadly driving down emissions at a nominally similar rate. However, while the preceding decade saw about a 25 percent reduction in absolute carbon dioxide emissions, a 40 to 50 percent absolute reduction is projected for the next decade.

The European Union has the most stringent requirements for fuel economy today. Car makers that fail to meet carbon dioxide limits after 2020 will face massive penalties. While carbon dioxide limits for commercial vehicles are still under development, the system for trucks will go much further than for cars, controlling every unit and component that impact fuel economy.

Different regulations and consequences for violations are under discussion for the key regions of the EU, United States and China. For example, China will probably take the initiative in major cities and might only allow full electric vehicles in certain urban areas.

Formulating FE Additives

Lowering lubricant viscosity is one measure being taken to reduce fuel consumption. However, the downside of this approach can be higher wear. An example is a field trial of a low-viscosity gear oil in 10 trucks totaling more than 500,000 kilometers. The gears showed increased wear with thinner oil, although part condition still met OEM requirements.

Ideally, hardware can be redesigned to compensate for the use of lower viscosity oils, but reformulation using additives at higher treat rates can also help offset the effects of lower viscosity. But OEMs and additive or oil companies have to work hand-in-hand from the start of a new development project to ensure reliable performance.

An example where this approach was followed successfully is Mercedes latest generation automatic transmission. Because the company worked closely with the lubricant supplier, it is able to use by far the thinnest ATF on the global market, gaining a 1.5 percent fuel economy benefit from just the fluid in the new hardware. It is a simple concept to grasp that lower viscosity equals fuel economy benefits, but the ability to both measure this value and to tailor the fluid to the engine and vehicle platform is more challenging.

For instance, Afton testing showed a 1.8 percent fuel economy benefit for a specific vehicle using an SAE 0W-16 engine oil. However, in a different vehicle, moving to lower and lower high-temperature high-shear viscosities showed the opposite effect.

Lower viscosity is not a bottomless well that we can use to access fuel economy benefits. Rather, low-viscosity fluids need to be matched to newer engine platforms specifically designed to run on the thinner oils.

As proof of this concept, consider that in the last two decades cars have become heavier and offer more comfort, safety and performance. At the same time, fuel economy has improved significantly. New hardware design, downsizing and a new category of lubricants developed in concert allowed this progress.

Lube & Additive Effects

Fuel economy effects can be differentiated by those derived directly from the lubricant and those produced by hardware changes, where the fluid plays a supporting role. Next generation lubricants cannot focus solely on fuel economy. They also have to account for any hardware changes on the horizon, which often are reflected in ACEA or OEM specifications.

In addition, fuel quality around the globe has to be considered, as well as increasing use of biofuels. Finally, considering only one vehicle component will not produce optimum benefits. Rather, the entire power train – including wheel bearings, hubs, etc. – has to be considered.

OEMs are adopting a number of hardware changes to meet fuel economy requirements, several of which place demands on future lubricant and additive technologies.

The priorities are clear, and we have to meet all market demands for new additive chemistry. To achieve this, we need the right R&D tools because nothing can be developed today without the use of statistical tools like design of experiments (DoE).

Besides that, testing is a key focus. Having meaningful screening tests, well working component tests and the right set up for field testing are critical for future development. All these factors should be included in a development strategy for new lubricants.

The table shows how different additive components support lubricant-impacted and hardware-impacted fuel economy improvements. Note that while antiwear components do not support fuel economy directly, the use of lower viscosity base oils requires careful selection of the antiwear to compensate for the higher wear potential.

The shift to steel pistons improves fuel economy because they allow higher combustion pressures to be applied, which improves performance. However, the oil and cooling water will run hotter, potentially increasing deposit formation in piston ring grooves. Hence, improved antioxidant performance is critical. Aftons testing of next generation antioxidants suggests that depending on the application, oil oxidation life can be extended by almost a factor of three.

Turbochargers play a key role in modern engine technology. Their use has led to far higher engine performance, allowing engines to be downsized while boosting output. Turbocharging is also crucial to reduce exhaust emissions; however, they are very sensitive to deposits.

Base oil composition can have a significant impact on turbocharger and piston deposits. A key rating of base oil quality is the Noack volatility test. Light base oil components with lower boiling points can potentially be transformed into the vapor phase in the engine, causing turbocharger, piston or oil mist separator deposits.

The Carnot cycle defines the maximum theoretical efficiency of a combustion engine. Further efficiency losses result mainly from friction in the hardware components. The largest amount of friction arises between the piston ring and the cylinder liner. Reducing this steel-on-steel friction is key to improving fuel economy. The SRV or pin-on-disc-test is a useful screen test to measure this friction.

Losses in the plain bearings are also considerable and are targeted for reduction. One concept is to replace them with roller bearings on the crankshaft. However, while roller bearings reduce friction losses, they can be adversely affected by combustion pressure surges.

Electric cars are probably the future of individual travel in passenger cars. While it is difficult to predict the future widespread adoption of electric vehicle, an EU guideline plans to prohibit registration of cars with conventional internal combustion engines by 2030.

As a consequence, more and more electrical and electronic components are being exposed to the lubricant. This applies mainly to driveline applications like transmissions or axles, but is also found in the engine. Hence, demands that were never made before have to be handled by the lubricant.

Higher combustion pressures, increased air intake into the engine, higher temperatures and other impacts have given rise to a phenomenon not detected for many years. These operating conditions increase the potential for autoignition, in this case low speed pre-ignition (LSPI), which can severely damage the engine if not controlled.

Last but not least, OEMs and customers still want to reduce the cost of ownership and are looking to extend oil drain intervals on top of all the challenges around fuel economy. For instance, OEMs want to increase drain intervals from 90,000 to 120,000 kilometers for commercial vehicle engine oils, 300,000 to 500,000 km for manual transmissions and 500,000 to 800,000 km for gear oils. These requirements place significant stress on all lubricant components.

Fuel Economy Testing

The general view in terms of testing is we could do better. Current testing protocols do what they were designed to do, but there is always room to improve. Current tests can be time consuming, produce a limited amount of data, can be inaccurate and have only regional application. Thus, they provide only limited information about how petroleum additives may enable new technologies.

Knowledge has advanced significantly in the period since the tests were set up. This is particularly true where lubricant research and evaluation is linked directly to vehicle emissions.

To help alleviate this situation, Afton developed the proprietary Afton Continuously Aging Fuel Economy (A-CAFE) test. This vehicle test allows formulators to evaluate multiple areas of influence in somewhat real world conditions.

A-CAFE is a chassis-dynamometer test based on a composite drive cycle. It monitors real time of fuel consumption. Because it runs continuously, the test generates a vast quantity of precise and repeatable fuel consumption data, compared to conventional fuel economy tests.

Fired engine tests are very complex and expensive because they burn larger quantities of fuel. In contrast, motored engine tests provide an elegant method to evaluate only frictional effects in an engine. In such tests, the engine is not driven by fuel, but by an external electric motor. Torque is measured with sensitive in line torque cells, and coolant and oil temperature are precisely controlled. A-CAFE enables researchers to explore fuel consumption as the oil ages in real time.

Conclusion

Fuel economy remains the key driver for all OEMs around the globe. However, only reducing viscosity provides limited benefits. To gain the optimal amount of fuel economy improvement, lubricants must be treated as hardware design elements and be developed in concert with new component designs.

To improve fuel economy in the future, the lubricant will become more important than today. OEMs will need to consider to involving additive and oil companies much earlier in the progress of development projects than they do today.

Walter Kudlich is OEM Relationship Director for Afton Chemical EMEAI, based in Hamburg, Germany. Contact him at walter.kudlich@aftonchemical.com.